The present invention relates to a drilling tool, particularly for working metals, in which a tool holder is provided with a receiving slot, which is open on three sides in its frontal or end surface, and an interchangeable drill blade is fastened in the receiving slot.
For deep drilling metallic materials, particularly steel materials, in addition to one-piece spiral drills, drilling tools are employed which comprise a drill rod and a drill bit inserted therein. In such a composite drilling tool, special requirements must be met with respect to stability as well as wear resistance of the drill bit.
For drilling deep holes where the ratio of hole diameter to hole depth may be 1:100, for example, deep hole drills are known in which a hard metal drill blade is soldered into a recess on the drill head. Deep hole drills have one cutting edge and have an asymmetrical design of the cutting edge with an eccentric cutting tip. The eccentrically arranged drill blade terminates substantially with the edge region of the drill head.
Due to the eccentric design of the cutting edge, it is necessary, particularly when starting to drill, to guide the drill head via a drill sleeve. Guide strips are also provided which prevent lateral escape of the drill head.
For the production of holes of a shallower depth, such deep hole drills cannot be used because the additional devices required for starting the hole, e.g., drill sleeves and guide strips, and for the intake and discharge of drilling oil are too complicated for this purpose. Moreover when the diameter of the drilled hole is to be changed, complete drill heads including the associated accessories must be interchanged.
Drilling tools have been proposed for producing holes of shallow depths in which a drill bit made of a high-speed steel is inserted into a receiving slot in a drill shaft or rod and is fastened by means of a screw. Such a drill generally operates with drill water, an emulsion of water and oil, to cool the cutting edge and to transport the borings out of the hole. When such holes are drilled without guides it is important that the drill bit have a stable position. In order to center and secure the drill bit in the drill shaft, the bit is provided with an external centering means in that the drill bit is provided with two centering protrusions in its outer region which enclose the drill shaft. This imparts a certain length to the drill bit. Such drilling tools are used in particular for holes with more than 20 mm diameter so that the drill bit is rather large. The drawback of such an embodiment is, inter alia, that the external centering means do not provide stable axial support.
The advancing force during drilling, particularly if the drill bit is significantly broader than the drill shaft, is absorbed only by an abutment in the order of magnitude of the diameter of the drill shaft. Additionally there exist considerable bending stresses in the bit due to the size of the bit with its external centering means. The high cutting forces occurring during deep drilling -- with a drill diameter of 40 mm, for example, the advancing force is about 11,000, N -- must be able to be transferred by the drill bit to the tool shaft so that optimum abutment and support of the drill bit are important. Finally, a relatively large amount of high-speed steel is required for a large drill blade bit which is distinguished by its toughness but is not wear resistant enough for use in a high performance drill.